Apparatus for and method of recording image

ABSTRACT

The intensity of a light beam is controlled accurately depending on an image to be recorded on a photosensitive medium. An apparatus for recording an image by scanning a photosensitive medium with a light beam generated based on an image signal has a recording duty cycle detector for detecting a recording duty cycle of an image to be recorded on the photosensitive medium based on the image signal, and a light beam intensity correcting memory for modulating the intensity of the light beam based on the detected recording duty ratio. Since the intensity of the light beam is modulated based on the recording duty ratio of the image recorded on the photosensitive medium, the amount of light of the light beam can be adjusted depending on the image actually recorded on the photosensitive medium.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for and a method ofrecording an image on a photosensitive medium by scanning thephotosensitive medium with a light beam.

[0003] 2. Description of the Related Art

[0004] There has heretofore been known an image recording apparatus forrecording an image on a photosensitive medium such as a film by scanningthe photosensitive medium with a light beam, as disclosed in Japaneselaid-open patent publication No. 63-191473, for example.

[0005] The above publication refers to the following problem: When ahalftone dot recorded on a film by a light beam by way of exposure isdeveloped, it usually becomes slightly greater than the recordedhalftone dot due to the Gaussian distribution of a light spot on thefilm. This phenomenon tends to be more noticeable when the developingliquid used to develop halftone dots is deteriorated. It has beendescribed in the publication that since the extent by which the halftonedot becomes larger remains substantially constant irrespective of thesize of the halftone dot, if the halftone dot is of a small size whosehalftone dot % (hereinafter referred to as halftone %) is 10% or less,the effect that the larger halftone dot has on an image representationis greater than if the halftone dot is of a size for a medium gradation.

[0006] To solve the above problem, the publication discloses a lightbeam intensity correcting circuit as shown in FIG. 18 of theaccompanying drawings. As shown in FIG. 18, the light beam intensitycorrecting circuit has a light beam intensity correcting means 8comprising a memory (memory table) 2, a D/A converter 4, and a bufferamplifier 6. A multigradation digital image signal a, e.g., a digitalimage signal a in 256 gradations (raging from 0 to 100 halftone %), issupplied to a comparator 10 and the memory 2 of the light beam intensitycorrecting means 8.

[0007] The comparator 10 compares a threshold signal, which is of avalue in the range from 0 to 256, from a threshold memory 12 with thedigital image signal a, and outputs a binary signal c, which is of ahigh level or a low level, indicative of the compared result. When thebinary signal c is of a high level, a switch 14 has its common contactshifted to connect the light beam intensity correcting means 8 to anoptical modulator 16, as shown.

[0008] The optical modulator 16 modulates the intensity of a laser beamdepending on an output signal f from the light beam intensity correctingmeans 8. The intensity-modulated laser beam outputted from the opticalmodulator 16 is applied to record an image on a film 18.

[0009] The memory 2 stores an amount-of-light control value for keepingthe amount of light of the laser beam at a constant level when thehalftone % is in a shadow and medium range, and an amount-of-lightcontrol value for progressively reducing the amount of light of thelaser beam when the halftone % is in a highlight range, i.e., 10% orless.

[0010] With the conventional light beam intensity correcting circuitshown in FIG. 18, the extent by which a halftone dot becomes greater inthe highlight range is reduced depending on the degree of correction ofthe intensity of the laser beam, resulting in a halftone dot smallerthan usual.

[0011] The above publication reveals that since the intensity of thelaser beam is controlled to reduce the amount of light of the laser beamin the highlight range, the extent by which a halftone dot becomesgreater in the highlight range is eliminated, resulting in an increasein the accuracy of the halftone dot representation.

[0012] However, the disclosed conventional image recording apparatus isdisadvantageous in that since the intensity of the laser beam iscorrected in a unit of halftone %, i.e., in a unit of the entirehalftone dot, depending on the gradation of the input image signal, theintensity of the laser beam is corrected identically irrespective of theshape of the halftone dot.

[0013] Recently, there have been developed various CPT (Computer ToPlate) exposure apparatus for performing a platemaking process bydirectly exposing a photosensitive printing plate to an image. Forexample, Japanese laid-open patent publication No. 2000-35673 disclosesan inner-drum plate setter and a lithographic printing plate. When sucha photographic printing plate is exposed to a halftone dot image by alaser beam, a new problem arises if the intensity of the laser beam iscontrolled to a small degree in a range of small halftone %.

[0014] The photographic printing plate comprises a support bodysupporting thereon a photosensitive layer where an area irradiated by alaser beam remains as a halftone dot image. As shown in FIG. 19 of theaccompanying drawings, the photographic printing plate is basicallyproduced from a photosensitive medium 32 which has a photosensitivelayer 21 of a photopolymer disposed on a support body 20 of a metal basesuch as of aluminum or the like, and a transparent overcoat layer 22disposed on the photosensitive layer 21 for blocking oxygen.

[0015] When light is applied to an area 23 of the photosensitive medium32, the area 23 is hardened. Thereafter, the photosensitive medium 32 isheated to accelerate the hardening of the area 23. After thephotosensitive medium 32 is heated, it is developed in an alkalinedeveloping liquid, and the area of the photosensitive medium 32 which isnot irradiated by the light is scraped off by a brush or the like,producing a printing plate 25 where the irradiated area 23 serves as animage area 24.

[0016] As a result of an experiment conducted on the photosensitivemedium 32, it has been found that if the image area 24 is not hardenough and is present as an isolated image area or a small image area,it tends to fall off while the photosensitive medium 32 is beingprocessed in the developing process.

[0017] It has also been found that in an image recording apparatus forscanning the photosensitive medium 32 directly with a light beam to forma halftone dot image according to an area modulation (area gradation)process, in order to sufficiently produce an image with small halftone%, i.e., a highlight image, and keep the highlight image with smallhalftone % sufficiently resistant to plate wear, it is necessary toexpose the photosensitive medium 32 to an increased amount of recordinglight for the highlight image, as indicated by a qualitativecharacteristic curve 26 in FIG. 20 of the accompanying drawings, unlikethe conventional arrangement shown in FIG. 18.

[0018] However, when the amount of recording light increases, halftonedots in the shadow range are clustered to reduce the number ofresolution points of halftone %, resulting in such a problem that ablack thin line within a highlight area and a white thin line within ashadow area have different thicknesses.

[0019] As indicated by a qualitative characteristic curve 27 in FIG. 21of the accompanying drawings, it has also been found that when theamount of recording light increases, the amount of flaring lightproduced around an image area increases to create a fog in a non-imagearea, resulting an image irregularity.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of the present invention to provide anapparatus for and a method of recording an image by accuratelycontrolling the intensity of a light beam depending on the image to berecorded.

[0021] Another object of the present invention is to provide anapparatus for and a method of recording an image by making a highlightimage recorded on a photosensitive medium, which comprises a supportbody supporting thereon a photosensitive layer where an area irradiatedby light remains as an image, sufficiently resistant to plate wear, andpreventing the image from suffering image irregularities.

[0022] Still another object of the present invention is to provide anapparatus for and a method of recording an image with a simplecorrective arrangement without the need for a corrective memory table.

[0023] According to the present invention, there is provided anapparatus for recording an image by scanning a photosensitive mediumwith a light beam generated based on an image signal, comprisingrecording duty cycle detecting means for detecting a recording dutycycle of an image to be recorded on the photosensitive medium based onthe image signal, and light beam intensity modulating means formodulating the intensity of the light beam based on the detectedrecording duty ratio.

[0024] Since the light beam intensity modulating means modulates theintensity of the light beam based on the recording duty ratio, detectedby the recording duty cycle detecting means, of the image to be recordedon the photosensitive medium, the amount of light of the light beam canbe adjusted depending on the image actually recorded on thephotosensitive medium.

[0025] If the photosensitive medium is of such a nature that an areairradiated with light remains as an image, then the light beam intensitymodulating means comprises means for modulating the intensity of thelight beam to a higher level in a highlight area of the image. The platewear resistance of the highlight area of the image on the photosensitivemedium can be kept at a sufficient level.

[0026] The highlight area preferably comprises a highlight area smallerthan 25% of all gradations of the image recorded on the photosensitivemedium.

[0027] The recording duty cycle detecting means may comprise a low-passfilter. With the low-pass filter used, it is not necessary to use acorrecting memory table.

[0028] The recording duty cycle detecting means may comprise means fordetecting a recording duty cycle corresponding to a given area in theimage recorded on the photosensitive medium. With this arrangement, itis not necessary to detect all duty ratios of the image, and theprocessing operation can be performed at a high speed.

[0029] If it is not necessary to detect all duty ratios of the image,then the apparatus may further comprise random number applying means forvarying the position of the given area in the image with a randomnumber, or random number applying means for varying the size of thegiven area in the image with a random number, or the light beamintensity modulating means may comprise random number applying means forapplying a random number to the detected recording duty cycle, and meansfor modulating the intensity of the light beam based on the recordingduty cycle to which the random number is applied by the random numberapplying means. In this manner, it is possible to eliminate beats thatmay occur between the given area for detecting the recording duty ratioand the recorded image.

[0030] If the light beam comprises a plurality light beams forsimultaneously scanning the photosensitive medium to record the imagethereon, then recording duty ratio detecting means may comprise aplurality of recording duty ratio detecting means associatedrespectively with images recorded on the photosensitive medium based onrespective image signals to generate the light beams, respectively, andthe light beam intensity modulating means may comprise a plurality oflight beam intensity modulating means associated respectively withrecording duty ratios detected by the recording duty ratio detectingmeans, respectively. With this arrangement, the intensities of the lightbeams can be modulated while eliminating beats.

[0031] If beats are allowed and the light beam may comprise a pluralityof light beams for simultaneously scanning the photosensitive medium torecord the image thereon, then the recording duty ratio detecting meansmay comprise means for determining an average recording duty ratio ofimages recorded on the photosensitive medium based on respective imagesignals to generate the light beams, respectively, and the light beamintensity modulating means may comprise means for modulating thebrightnesses of the light beams based on the average recording dutyratio.

[0032] According to the present invention, there is also provided anapparatus for recording an image by scanning a photosensitive mediumwhich is fed in an auxiliary scanning direction, with a light beamgenerated based on an image signal in a main scanning directionsubstantially perpendicular to the auxiliary scanning direction,comprising present recording duty cycle detecting means for detecting apresent recording duty cycle of an image to be recorded on thephotosensitive medium based on the image signal, light beam intensitymodulating means for modulating the intensity of the light beam based onthe detected present recording duty ratio, preceding recording dutycycle detecting means for detecting a preceding recording duty cycle ofthe image at a position scanned later than the present recording dutycycle detecting means in the main scanning direction, intensitymodulation correcting means for comparing the detected precedingrecording duty cycle and the detected present recording duty cycle tocorrect the modulation of the intensity of the light beam with the lightbeam intensity modulating means.

[0033] The preceding recording duty cycle detecting means detects apreceding recording duty cycle of the image at a position scanned laterthan the present recording duty cycle detecting means in the mainscanning direction. The intensity modulation correcting means comparesthe detected preceding recording duty cycle and the detected presentrecording duty cycle with each other to correct the modulation of theintensity of the light beam with the light beam intensity modulatingmeans. Thus, the intensity of the light beam can be corrected finelydepending on details of the image.

[0034] If the photosensitive medium is of such a nature that an areairradiated with light remains as an image, then the light beam intensitymodulating means may comprise means for modulating the intensity of thelight beam to a higher level in a highlight area which is smaller than25% of all gradations of the image.

[0035] If the preceding recording duty cycle is of a value correspondingto the highlight area which is smaller than 25% of all gradations of theimage, and the present recording duty cycle is of a value correspondingto an area except the highlight area which is smaller than 25% of allgradations of the image, then the intensity modulation correcting meansmay comprise means for correcting the modulation of the intensity of thelight beam to cause the intensity of the light beam to return from agiven position in the highlight area to a normal intensity. With thisarrangement, if the image includes a highlight area and an area otherthan a highlight area arranged forward in the main scanning direction,then it is possible to prevent the density from being stepped due to anincrease in the amount of light applied to the image other than thehighlight area.

[0036] According to the present invention, there is also provided amethod of recording an image by scanning a photosensitive medium with alight beam generated based on an image signal, comprising the steps ofdetecting a recording duty cycle of an image to be recorded on thephotosensitive medium based on the image signal, and modulating theintensity of the light beam based on the detected recording duty ratio.

[0037] Because the intensity of the light beam is modulated based on thedetected recording duty ratio, the amount of light of the light beam canbe adjusted depending on the image actually recorded on thephotosensitive medium.

[0038] If the photosensitive medium is of such a nature that an areairradiated with light remains as an image, the step of modulating theintensity of the light beam may comprise the step of modulating theintensity of the light beam to a higher level in a highlight area whichis smaller than 25% of all gradations of the image. Therefore, the platewear resistance of the highlight area of the image on the photosensitivemedium can be kept at a sufficient level.

[0039] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich preferred embodiments of the present invention are shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a perspective view of a printing plate producing system;

[0041]FIG. 2 is a schematic perspective view, partly in block form, ofan image recording apparatus in the printing plate producing systemshown in FIG. 1;

[0042]FIG. 3 is a block diagram of an exposure signal controller in theimage recording apparatus;

[0043]FIG. 4 is a diagram illustrative of an interlaced scanning processfor simultaneously recording three scanning lines;

[0044]FIG. 5 is a diagram showing an amount-of-light controlcharacteristic curve;

[0045]FIG. 6 is a diagram showing an image density characteristic curve;

[0046]FIG. 7 is a diagram showing a determining region;

[0047]FIG. 8 is a block diagram of another exposure signal controller;

[0048]FIG. 9 is a block diagram of still another exposure signalcontroller;

[0049]FIG. 10 is a diagram illustrative of the generation of beats;

[0050]FIG. 11 is a block diagram of yet another exposure signalcontroller;

[0051]FIG. 12 is a diagram showing other amount-of-light controlcharacteristics;

[0052]FIG. 13 is a diagram illustrative of a tailing phenomenon;

[0053]FIG. 14 is a block diagram of an exposure signal controller foreliminating a tailing phenomenon;

[0054]FIG. 15 is a diagram illustrative of a process of eliminating atailing phenomenon;

[0055]FIG. 16 is a block diagram of yet still another exposure signalcontroller;

[0056]FIG. 17 is a block diagram of a further exposure signalcontroller;

[0057]FIG. 18 is a block diagram of a conventional image recordingapparatus;

[0058]FIG. 19 is a cross-sectional view of a photosensitive medium wherean area exposed to light remains as an image;

[0059]FIG. 20 is a diagram illustrative of the plate wear resistance ofa photosensitive medium; and

[0060]FIG. 21 is a diagram illustrative of the image irregularities of aphotosensitive medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Like or corresponding parts are denoted by like or correspondingreference characters throughout views.

[0062]FIG. 1 shows in perspective a printing plate producing system 30which incorporates an apparatus for and a method of recording an imageaccording to the present invention. The printing plate producing system30 is a CTP (Computer To Plate) system for producing a printing plate onwhich an image for generating a printed material is recorded, directlyfrom digital image data without using a film.

[0063] As shown in FIG. 1, the printing plate producing system 30basically comprises a plate supplying device 34 for supplying anunexposed photosensitive medium 32, an image recording apparatus 36 forscanning an unexposed photosensitive medium 32 with a light beam Lmodulated by an image signal to record an image on the photosensitivematerial 32, and a developing device 38 for developing the imagerecorded on the photosensitive medium 32.

[0064] The photosensitive medium 32 is the same as the photosensitivemedium 32 as shown in FIG. 19 which comprises the support body 20supporting thereon the photosensitive layer 21 where an area irradiatedby light remains as the image area 24.

[0065] The plate supplying device 34 holds a plurality of unexposedphotosensitive mediums 32 and supplies one at a time of the unexposedphotosensitive mediums 32 to the image recording apparatus 36 in thedirection indicated by the arrow. The image recording apparatus 36 feedsthe unexposed photosensitive medium 32 supplied from the plate supplyingdevice 34 with an exposure stage 40 in an auxiliary scanning directionindicated by the arrow Y, and at the same time scans the unexposedphotosensitive medium 32 with a laser beam L, which has been modulatedby an image signal supplied from an image recording unit 42, in a mainscanning direction indicated by the arrow X that is perpendicular to theauxiliary scanning direction, thereby recording a two-dimensional image,i.e., a halftone dot image, on the photosensitive medium 32 according toan area modulating process. The developing device 38 develops the imagerecorded on the photosensitive medium 32 that is supplied from the imagerecording apparatus 36.

[0066]FIG. 2 shows in perspective the image recording apparatus 36together with its control circuit shown in block form.

[0067] As shown in FIG. 2, the image recording apparatus 36 has arecording light source 46 energizable by a laser driver 44 foroutputting a light beam L as a laser beam for recording an image on aphotosensitive medium 32, and a synchronizing light source 50energizable by a laser driver 48 for outputting a synchronizing laserbeam S for generating a synchronizing clock signal used when the lightbeam L scans the photosensitive medium 32 in the main scanningdirection.

[0068] The image recording apparatus 36 includes a mechanical shutter52, a variable-transmittance ND filter 54, an acousto-optic modulator(AOM) 56, a resonant scanner 58 as a light beam deflecting means, ascanning lens 59, and reflecting mirrors 60, 62 which are successivelydisposed in the light path of the light beam L that is outputted fromthe recording light source 46.

[0069] The mechanical shutter 52 is movable into and out of the lightpath of the light beam L by a displacing unit 64 for selectivelysupplying and blocking the light beam L to the photosensitive medium 32.The variable-transmittance ND filter 54 is positionally variable withrespect to the light path of the light beam L by a displacing unit 66,for controlling the amount of light of the light beam L.

[0070] The AOM 56, which functions as a light beam intensity modulatingmeans, turns on and off the light beam L depending on an image to berecorded on the photosensitive medium 32. Image data for recording animage on the photosensitive medium 32 are read from an image memory 68,and converted into an on/off modulation signal by an exposure signalcontroller 70. The on/off modulation signal is supplied to an AOM driver72. The AOM driver 72 supplies the AOM 56 with a drive signal which is acombination of signals having different frequencies, each turned on andoff depending on the image data. Therefore, the light beam L is turnedon and off depending on the image data by the AOM 56, and divided into aplurality of light beams L depending on the frequencies, which aresupplied to the resonant scanner 58.

[0071] The resonant scanner 58 oscillates a mirror at a high speed witha drive signal supplied from a scanner driver 74, and deflects the lightbeam L from the AOM 56 in the main scanning direction X and supplies thedeflected light beam L to the scanning lens 59. The light beam L thathas passed through the scanning lens 59 is adjusted in its scanningspeed with respect to the main scanning direction, and is then reflectedby the reflecting mirrors 60, 62 toward the photosensitive medium 32.

[0072] A reflecting mirror 78 movable into and out of the light path ofthe light beam L by a displacing unit 76 is disposed between thereflecting mirror 62 and the photosensitive medium 32. When thereflecting mirror 78 is positioned in the light path of the light beamL, it reflects the light beam L to a photosensor 80 for monitoring anamount of light of the light beam L. The photosensor 80 detects anamount of light of the light beam L, which is converted by an A/Dconverter 82 into a digital signal that is supplied to a CPU, not shown.

[0073] The resonant scanner 58, the scanning lens 59, the reflectingmirror 60, a reflecting mirror 84, a reference grating 86, a light guiderod 88, and photosensors 90 a, 90 b are successively disposed in thelight path of the synchronizing laser beam S that is outputted from thesynchronizing light source 50.

[0074] The synchronizing light source 50 is positioned to apply thesynchronizing laser beam S to the resonant scanner 58 at an angledifferent from the laser beam L. The synchronizing laser beam S isreflected and deflected in main scanning direction indicated by thearrow X by the resonant scanner 58. The synchronizing laser beam Sdeflected by the resonant scanner 58 travels through the scanning lens59 to the reflecting mirror 60. The synchronizing laser beam S isreflected by the reflecting mirror 60 toward the reflecting mirror 84,which reflects the synchronizing laser beam S toward the referencegrating 86. The synchronizing laser beam S passes through the referencegrating 86.

[0075] The reference grating 86 is elongate in the main scanningdirection X, and has a linear succession of slits 92 along itslongitudinal direction, the number of the slits depending on theresolution.

[0076] The light guide rod 88, which is substantially cylindrical inshape, is disposed behind the reference grating 86 to receive thesynchronizing laser beam S that has passed through the reference grating86. The light guide rod 88 is made of a material capable of transmittinglight therethrough. The synchronizing laser beam S that has entered thelight guide rod 88 is repeatedly reflected therein and travelstherethrough to the photosensors 90 a, 90 b which are disposed on therespective ends of the light guide rod 88.

[0077] To the photosensors 90 a, 90 b, there is connected a gratingclock generator 94 for generating a grating clock signal GCLK comprisingas many clock pulses as the number of the slits 92 of the referencegrating 86 in each scanning cycle. The grating clock signal GCLKgenerated by the grating clock generator 94 is supplied, as a recordingtiming signal for the image data to be recorded with respect to the mainscanning direction X, to the exposure signal controller 70.

[0078] The photosensitive medium 32 is positioned on and held by theexposure stage 40, which can be fed in the auxiliary scanning directionY by a ball screw 100 that is rotatable about its own axis by anauxiliary scanning motor 98. The auxiliary scanning motor 98 isenergizable by an auxiliary scanning motor driver 104 based on a motordriving reference clock signal that is supplied from an auxiliaryscanning motor driving clock generator 102. The motor driving referenceclock signal is generated by the auxiliary scanning motor driving clockgenerator 102 based on a scanning clock signal which is a main scanningstart timing signal supplied from the scanner driver 74.

[0079]FIG. 3 shows a circuit including the exposure signal controller 70and the AOM driver 72 according to a first embodiment of the presentinvention.

[0080] As shown in FIG. 3, the exposure signal controller 70 has atiming signal generator 150 for generating various timing signals orclock signals such as a pixel clock (dot clock) signal PCLK, a wordclock signal WCLK for every 16 pixels (16 bits), and a main scanningarea clock signal XSYNC for determining a main scanning area on thephotosensitive medium 32, in synchronism with the grating clock signalGCLK supplied from the grating clock generator 94.

[0081] Of those timing signals, the main scanning area clock signalXSYNC and the word clock signal WCLK are supplied to a line memorycontroller 152.

[0082] The line memory controller 152 basically controls reading andwriting processes of six line memories 201-206 under the control of aCPU 148 as a control circuit or control means. The line memorycontroller 152 controls the six line memories 201-206, three at a time,to toggle between the reading and writing processes in synchronism withthe main scanning area clock signal XSYNC.

[0083] Prior to recording an image on the photosensitive medium 32, eachof the line memories 201-206 is supplied with one main scanning line ofimage data, i.e., binary on/off image data composed of 54400 pixels ordots, from the image memory 68 whose reading and writing processes arecontrolled by the CPU 148 via an input line memory selector 207 in thewriting process controlled by the line memory controller 152.

[0084] In an image recording operation, while image data are beingsupplied from the image memory 68 to one bank of line memories 201-203(or 204-206), new image data are supplied from the other bank of linememories 204-206 (or 201-203) via an output line memory selector 208 tothree parallel-to-serial converters 209. For an easier understanding,the three parallel-to-serial converters 209 are shown as oneparallel-to-serial converter.

[0085] In synchronism with the word clock signal WCLK and the pixelclock signal PCLK, the parallel-to-serial converters 209 outputrespective binary image signals IS1-IS3 each representing serial datahaving a value 0 (low level) or a value 1 (high level) to the AOM driver72.

[0086] The AOM driver 72 comprises signal generators 211-213 forgenerating respective signals f1-f3 having a constant amplitude andrespective different high frequencies f1-f3, multipliers 221-223 formultiplying the binary image signals IS1-IS3 by the respective signalsf1-f3, a combiner 226 for combining product signals from the multipliers221-223, a voltage control gain varying circuit 218 for adjusting theamplitude of a combined signal outputted from the combiner 226, and anamplifier 228 for amplifying and applying the amplitude-adjustedcombined signal to a control terminal of the AOM 56.

[0087] Based on the frequencies f1-f3 of the signal outputted from theamplifier 228, the AOM 56 processes the single light beam L into threelight beams L that are offset on the photosensitive medium 32 in theauxiliary scanning direction Y, intensity-modulates and outputs thethree light beams L based on the amplitude of the output signal from theamplifier 228.

[0088] Therefore, the three light beams L are turned on and off insynchronism with the values 1, 0 (on and off) of the binary imagesignals IS1-IS3.

[0089]FIG. 4 shows an interlaced scanning process for simultaneouslyrecording images of three scanning lines that are offset in theauxiliary scanning direction Y on the photosensitive medium 32. First,the interlaced scanning process for simultaneously recording threescanning lines will be described below.

[0090] In an auxiliary scanning position Y1, images of first and thirdscanning lines, indicated by scanning line numbers in FIG. 4, arerecorded on the photosensitive medium 32 with two light beams L based onthe image data stored in the line memories 202, 203. In this scanningcycle, all image data stored in the line memory 201 are set to 0.

[0091] Then, the photosensitive medium 32 is fed stepwise a distance ΔYin the auxiliary scanning direction to a position Y2 (=Y1+ΔY) by theauxiliary scanning motor 98. In this position Y2, images of second,fourth, and sixth scanning lines are recorded on the photosensitivemedium 32 with three light beams L based on the image data stored in theline memories 204, 205, 206.

[0092] Thereafter, the photosensitive medium 32 is fed again stepwise adistance ΔY in the auxiliary scanning direction to a position Y3(=Y2+ΔY=Y1+2ΔY) by the auxiliary scanning motor 98. In this position Y3,images of fifth, seventh, and ninth scanning lines are recorded on thephotosensitive medium 32 with three light beams L based on the imagedata stored in the line memories 201, 202, 203.

[0093] While the images are being recorded on the photosensitive medium32 based on the image data stored in one bank of the line memories201-203 or 204-206, new image data are supplied from the image memory 68to the other bank of the line memories 204-206 or 201-203. In thismanner, the line memories 201-206 are operated in a toggled fashion,i.e., one bank at a time, to record images scanning lines on thephotosensitive medium 32.

[0094] The above toggled image recording operation is repeated until adesired two-dimensional image is recorded in a desired area on thephotosensitive medium 32.

[0095] Information as to which scanning line's image data are to bestored in which of the line memories 201-206 is stored in a memory inthe CPU 148, and the line memory controller 152 controls the linememories 201-206 based on the stored information.

[0096] The toggled operation of the line memories 201-206 and theinterlaced scanning process for simultaneously recording images of threescanning lines shown in FIG. 4 make it possible to shorten a period oftime required to record a two-dimensional image on the photosensitivemedium 32. However, the principles of the present invention are notlimited to the interlaced scanning process for simultaneously recordingimages of three scanning lines.

[0097] In FIG. 3, when an image is recorded on the photosensitive medium32, image data corresponding to a given area in the image are suppliedfrom the line memories 201-206 via the line memory selector 208 to arecording duty ratio detector 154 as a recording duty ratio detectingmeans which is controlled by the line memory controller 152.

[0098] The recording duty ratio detector 154 which is also controlled bythe CPU 148 detects the number of data that are turned on, i.e., thenumber of pixels having a value 1, in the given area in the image,determines the percentage of the number of turned-on data (the number ofrecording pixels of an image area in the given area) in the number ofall pixels in the given area, or a recording duty ratio PDUTY defined bythe percentage, and supplies the recording duty ratio PDUTY to a lightbeam intensity correcting memory (also referred to as“amount-of-exposure control memory”) 156 that is controlled by the CPU148.

[0099] The light beam intensity correcting memory 156 operates as alight beam intensity modulating means for modulating the intensity ofthe light beam L based on the recording duty ratio PDUTY determined bythe recording duty ratio detector 154.

[0100]FIG. 5 shows an amount-of-light control characteristic curve 158that is set in the light beam intensity correcting memory 156. Theamount-of-light control characteristic curve 158 can be modified asdesired by the CPU 148.

[0101] In FIG. 5, the horizontal axis represents the recording dutyratio PDUTY in % and the vertical axis represents the amount ofrecording light P. In the example shown in FIG. 5, when the recordingduty ratio PDUTY is in a range from 0 to 6%, the amount of recordinglight P is set to a level Lh (also referred to as “highlight level Lh”)for increasing the intensity of the light beam L. When the recordingduty ratio PDUTY is in a range from 6 to 25%, the amount of recordinglight P is set to a level that linearly decreases from the highlightlevel Lh to a medium level Lm. When the recording duty ratio PDUTY is ina range from 25 to 100%, the amount of recording light P is set to themedium level Lm.

[0102] Since the recording duty ratio PDUTY is determined as thepercentage of the number of turned-on data (the number of recordingpixels of an image area in the given area) in the number of all pixelsin the given area, as described in detail later on, the amount-of-lightcontrol characteristic curve 158 is established such that the amount ofrecording light P is greater in a highlight area where the image isbrighter, than in medium and shadow areas.

[0103] The recording duty ratio PDUTY represents the number of recordingpixels that make up the image area in the given area. The horizontalaxis in FIG. 5 may be considered as representing gradations from 0 to100% of the image or halftone dot % of the halftone dot image.

[0104]FIG. 6 shows an image density characteristic curve 160 of an imagethat has been developed by the developing device 38 on thephotosensitive medium 32. In FIG. 6, the horizontal axis represents theamount of recording light P on a logarithmic scale. The photosensitivemedium 32 produces binary image densities when the amount of recordinglight P is of the zero level and the amount of recording light P is ofthe medium level Lm or the highlight level Lh.

[0105] It can be seen from the image density characteristic curve 160that an image density Dh produced at the highlight level Lh is greaterthan an image density Dm produced at the medium level Lm.

[0106] In FIG. 3, the light beam intensity correcting memory 156 outputsan intensity correcting digital signal according to the amount-of-lightcontrol characteristic curve 158 shown in FIG. 5 to a D/A converter 162,which converts the intensity correcting digital signal into an intensitycorrecting analog signal Sp. The intensity correcting analog signal Spis supplied to a voltage control input terminal of the voltage controlgain varying circuit 218 of the AOM driver 72. The voltage control gainvarying circuit 218 adjusts a gain to a value proportional to themagnitude of the intensity correcting analog signal Sp.

[0107] As a result, the amplitude of the combined signal outputted fromthe combiner 226 is adjusted by the voltage control gain varying circuit218. The amplitude-adjusted signal is applied from the voltage controlgain varying circuit 218 to the amplifier 228, which applies anamplified output signal to the AOM 56 to adjust the intensity of thelight beam L outputted therefrom.

[0108] The printing plate producing system 30 is basically constructedas described above. Operation of the printing plate producing system 30will now be described below with reference to FIG. 2.

[0109] When the printing plate producing system 30 is turned on, thescanner driver 74 supplies a drive signal to the resonant scanner 58 tostart oscillating its mirror at a high speed.

[0110] The laser driver 48 supplies a drive signal to the synchronizinglight source 50, which outputs a synchronizing light beam S. Thesynchronizing light beam S outputted from the synchronizing light source50 is reflected and deflected by the resonant scanner 58, and guided bythe scanning lens 59 and the reflecting mirrors 60, 84 to the referencegrating 86.

[0111] The synchronizing laser beam S applied to the reference grating86 successively passes through the slits 92 as the synchronizing laserbeam S moves along the reference grating 86 in the main scanningdirection X, and enters as a pulsed light signal into the light guiderod 88. The pulsed synchronizing laser beam S is repeatedly reflected inthe light guide rod 88 and travels therethrough to the photosensors 90a, 90 b on the respective ends of the light guide rod 88. Thephotosensors 90 a, 90 b convert the pulsed synchronizing laser beam Sinto an electric signal and supplies the electric signal to the gratingclock generator 94.

[0112] The grating clock generator 94 shapes the waveform of theelectric signal and multiplies its frequency thereby to generate agrating clock signal GCLK. The grating clock signal GCLK generated bythe grating clock generator 94 is supplied to the exposure signalcontroller 70.

[0113] The exposure signal controller 70 converts image data read fromthe image memory 68 into an on/off modulated signal according to thesupplied grating clock signal GCLK and clock signals whose frequenciesare multiples of the frequency of the grating clock signal GCLK, andsupplies the on/off modulated signal to the AOM driver 72. The AOMdriver 72 supplies a drive signal which is a combination of signalshaving different frequencies, each turned on and off depending on theimage data, to the AOM 56.

[0114] The recording light source 46 energized by the laser driver 44outputs a light beam L for recording an image.

[0115] The light beam L is guided to the AOM 56 via thevariable-transmittance ND filter 54 which has been adjusted by thedisplacing unit 66 to produce a light beam L having a predeterminedamount of light. In the image recording process, the mechanical shutter52 that is positioned in front of the variable-transmittance ND filter54 is retracted out of the light path of the light beam L by thedisplacing unit 64.

[0116] The light beam L that is applied to the AOM 56 is turned on andoff by the AOM 56 depending on the image data, and divided into aplurality of light beams having different frequencies, which are thensupplied from the AOM 56 to the resonant scanner 58. The resonantscanner 58 reflects and deflects the light beams L, which are guided bythe scanning lens 59 and the reflecting mirrors 60, 62 to thephotosensitive medium 32.

[0117] The scanner driver 74 also supplies a scanning clock signal SCANgenerated in each main scanning cycle to the auxiliary scanning motordriving clock generator 102. Based on the supplied scanning clock signalSCAN, the auxiliary scanning motor driving clock generator 102 generatesand supplies a motor driving reference clock signal to the auxiliaryscanning motor driver 104. Based on the supplied motor driving referenceclock signal, the auxiliary scanning motor driver 104 generates a drivesignal and applies the drive signal to energize the auxiliary scanningmotor 98, which rotates the ball screw 100 about its own axis. Theexposure stage 40 is now displaced in the auxiliary scanning direction Yin synchronism with the scanning clock signal SCAN.

[0118] Therefore, the light beam L modulated with the image data isapplied to the photosensitive medium 32 in the main scanning direction Xwhile the photosensitive medium 32 is being fed in the auxiliaryscanning direction Y, thereby forming a two-dimensional image on theprinting plate 32. The photosensitive medium 32 with the two-dimensionalimage formed thereon is delivered to the developing device 38, whichdevelops the image recorded on the photosensitive medium 32. Thereafter,the photosensitive medium 32 is fed to a printing process.

[0119] Operation of an assembly including the recording duty ratiodetector 154 incorporated in the printing plate producing system 30 willbe described below.

[0120]FIG. 7 schematically shows binary image data stored in somestorage areas of the line memories 201-206. The line memories 201-206which are shown in FIG. 7 contain image data representing four pixels tobe blackened, i.e., turned-on pixels, only in those storage areas atcoordinates that are indicated by main scanning lines i3, i4 arrayed inthe auxiliary scanning direction Y and columns j4, j5 arrayed in themain scanning line X, and contain image data representing other pixelsto be unblackened, i.e., turned-off pixels.

[0121] When main scanning lines i1, i3, i5 are to be simultaneouslyrecorded based on image data recorded in the line memories 201-203 bythe three light beams L, a determining area (also referred to as“detecting area”) Darea is determined for detecting a recording dutyratio PDUTY corresponding to a given area in an image to be recorded onthe photosensitive medium 32. In the example shown in FIG. 7, thedetermining area Darea is set to a two-dimensional area of 60 pixelswhich are made up of 3 pixels arrayed in the auxiliary scanningdirection Y×20 pixels arrayed in the main scanning direction X.

[0122] First, the determining area Darea is established with respect tothe pixel data of i1×(j1−j20), i3×(j1−j20), i5×(j1−j20) at a leadingposition in the main scanning position X. A recording duty ratio PDUTYin the established determining area Darea, i.e., an average recordingduty ratio PDUTY, is detected as PDUTY=(2/60)×100=3.3%.

[0123] The recording duty ratio PDUTY (=3.3%) detected by the recordingduty ratio detector 154 is supplied to the light beam intensitycorrecting memory 156. In the light beam intensity correcting memory156, the amount-of-light control characteristic curve 158 shown in FIG.5 is referred to, and an amount of recording light P for the determiningarea Darea is selected as the highlight level Lh (P=Lh). The magnitudeof the intensity correcting analog signal Sp supplied from the D/Aconverter 159 to the voltage control gain varying circuit 218 is now setto a value corresponding to the highlight level Lh.

[0124] Therefore, an area on the photosensitive medium 32 whichcorresponds to the determining area Darea is exposed to the amount ofrecording light P at the highlight level Lh, with the result that theimage density in that area becomes a high density Dh (see FIG. 6).

[0125] The recording duty ratio detector 154 then establishes adetermining area Darea for the pixel data i×(j21−j40), i3×(j21−j40),i5×(j21−j40) at a next position in the main scanning position X, anddetects a recording duty ratio PDUTY in the established determining areaDarea. Based on the detected recording duty ratio PDUTY, the light beamintensity correcting memory 156 determines an amount of recording lightP in the determining area of i1×(j21−j40), i3×(j21−j40), i5×(j21−j40).

[0126] After the scanning of the photosensitive medium 32 with the mainscanning lines i1, i3, i5 is finished, the recording duty ratio detector154 establishes a determining area Darea for the pixel datai4×(j21−j40), i6×(j21−j40), i8×(j21−j40) for next main scanning linesi4, i6, i8, and detects a recording duty ratio PDUTY in the establisheddetermining area Darea.

[0127] Thereafter, recording duty ratios PDUTY are successivelydetected, amounts of recording light P are determined based on thedetected recording duty ratios PDUTY, and images are recorded on thephotosensitive medium 32.

[0128] Actually, the time when the recording duty ratio detector 154detects the recording duty ratio PDUTY is slightly out of phase with theimage signals IS1-IS3 that are supplied via the parallel-to-serialconverters 209 to the AOM driver 72. However, if it is taken intoconsideration that one halftone dot comprises 200 pixels×200 pixels,then a practically sufficient level of accuracy is achieved.

[0129] A recording duty ratio PDUTY can also be determined as a movingaverage. Specifically, a recording duty ratio PDUTY is detected in thedetermining area Darea with respect to the pixel data of i1×(j1−j20),i3×(j1−j20), i5×(j1−j20), and then a recording duty ratio PDUTY isdetected in the determining area Darea with respect to the pixel data ofi1×(j2−j21), i3×(j2−j21), i5×(j2−j21). A moving average may not bedetermined based on every single pixel, but may be determined based onevery plural pixels.

[0130] In the exposure signal controller 70 shown in FIG. 3, an averagerecording duty ratio PDUTY in an area of 60 pixels is detected toenergize the AOM driver 72. FIG. 8 shows a modified exposure signalcontroller 70B which has three recording duty cycle detectors 154A-154Cfor successively detecting respective recording duty cycles PDUTY eachfor 20 pixels in the main scanning direction. The light beam intensitycorrecting memory 156 produces amounts of recording light P with respectto the respective light beams L, which are converted by respective D/Aconverters 159A-159C into intensity correcting analog signals Sp thatcontrol the gains of three voltage control gain varying circuits218A-218C. Gain-controlled signals outputted from the voltage controlgain varying circuits 218A-218C are combined by a combiner 226A. In thismanner, the amounts of recording light P of the three light beams L canindividually be adjusted.

[0131] As shown in FIG. 9, the principles of the present invention arealso applicable to still another exposure signal control circuit 70C forcontrolling a single light beam L. The intensity of the signal lightbeam L may be modulated by an EOM (Electro-Optic Modulator) whichoperates based on an electro-optic effect that a refractive index varieswhen an electric field is applied, rather than being modulated by theAOM 56.

[0132] Various modifications of the present invention will be describedbelow.

[0133] Modification 1:

[0134] Elimination of possible beats caused by an interference betweenthe halftone dot pattern of a halftone dot image signal and a given area(an area of 3 pixels×20 pixels in FIG. 7) in which a recording dutycycle PDUTY is determined.

[0135] It has been found that if the size of a given area for detectinga recording duty cycle PDUTY, i.e., the size of a determining area Dareafor determining whether the halftone % of an image to be recorded on thephotosensitive medium 32 is in a highlight range or a medium range, issmaller than a halftone dot size (in the above embodiment, the halftonedot size is about 200 pixels×200 pixels whereas the size of thedetermining area Darea is 3 pixels×20 pixels), then the area of themedium halftone %, where the amount of light should not be increased, isrecognized in error as a highlight area, and its amount of light isincreased, and if that phenomenon occurs periodically, then itinterferes with the halftone dot pattern, resulting in beats that arevisually perceived.

[0136]FIG. 10 shows beats that occur between pixels recorded on thephotosensitive medium 32 and halftone dots thereon. In FIG. 10, pixelsPh shown hatched have an image density equal to the image density Dh atthe highlight level Lh, and pixels Pm shown blank have an image densityequal to the image density Dm at the medium level Lm. Other areas inFIG. 10 are free of pixels.

[0137] In FIG. 10, all eight pixels that make up each of halftone dotsin a vertical array including halftone dots 250 are blackened, and threeof eight pixels that make up each of halftone dots in vertical arraysincluding halftone dots 252 positioned adjacent to the halftone dots 250are blackened. As can be seen from FIG. 10, an image 253 which iscomposed of these halftone dots suffers periodic image densityirregularities, i.e., beats.

[0138] In order to prevent beats from occurring, the size of thedetermining area Darea may be set to a size equal to or greater than ahalftone dot area. However, the size of the determining area Darea thusset requires an increased amount of calculations, posing new problems inthat resources such as the CPU 148, the memories, etc. need to operateat higher speeds or the power supply needs to be increased in capacity.

[0139]FIG. 11 shows an exposure signal controller 70D arranged to avoidbeats without the need for an increased size for the determining areaDarea. As shown in FIG. 11, the exposure signal controller 70D includes,in addition to the components of the exposure signal controller 70 shownin FIG. 3, a random number generator 260 whose timing of generating arandom number is controlled by the line memory controller 152. A randomnumber Rd generated by the random number generator 260 is added by anadder 262 to a recording duty ratio PDUTY detected by the recording dutyratio detector 154. The adder 262 supplies the sum signal representingthe recording duty ratio PDUTYr with the random number added to thelight beam intensity correcting memory 156. The random number Rd may beRd=−1, 0, 1 (or −2, −1, 0, +1, +2).

[0140] The exposure signal controller 70D thus arranged is capable ofavoiding beats even if the determining area Darea is smaller than ahalftone dot area.

[0141] Rather than modulating the intensity of the light beam L based onthe recording duty ratio PDUTYr which is the sum of the recording dutyratio PDUTY and the random number Rd, the line memory controller 152 mayhave an address/size random number applying means 264 for varying areadout address with a random number thereby to vary the position of thedetermining area Darea in the image with the random number or to varythe size of the determining area Darea in the image with the randomnumber. Such an alternative arrangement is also capable of avoidingbeats.

[0142] The exposure signal controller 70D shown in FIG. 11 can reduceperiodic irregularities in the image to avoid beats caused by aninterference with the halftone dot pattern.

[0143] Modification 2:

[0144] Elimination of an inversion of the gradation.

[0145] As shown in FIG. 5, the amount-of-light control characteristiccurve 158 that is set in the light beam intensity correcting memory 156is established such that when the recording duty ratio PDUTY is in arange from 0 to 6%, the amount of recording light P is set to ahighlight level Lh for increasing the intensity of the light beam L,when the recording duty ratio PDUTY is in a range from 6 to 25%, theamount of recording light P is set to a level that linearly decreasesfrom the highlight level Lh to a medium level Lm, and when the recordingduty ratio PDUTY is in a range from 25 to 100%, the amount of recordinglight P is set to the medium level Lm.

[0146]FIG. 12 shows a stepped amount-of-light control characteristiccurve 158A that is established such that when the recording duty ratioPDUTY is in a range from 0 to 6%, the amount of recording light P is setto a highlight level Lh, and when the recording duty ratio PDUTY is in arange exceeding 6%, the amount of recording light P is set to a mediumlevel Lm.

[0147] With the stepped amount-of-light control characteristic curve158A, it is possible to reduce the storage capacity of the light beamintensity correcting memory 156. However, there is a possibility that anincrease in the halftone % due to an increase in the area of the imagearea 24 (see FIG. 19) upon an increase in the amount of light may exceedan increase in the halftone % in the image data, resulting in agradation inversion phenomenon.

[0148] To avoid such a gradation inversion phenomenon, the recordingduty cycle PDUTY, i.e., the increase in the amount of light with respectto the detected halftone %, should not be as sharp as shown in FIG. 12,but may be set to the amount-of-light control characteristic curve 158shown in FIG. 5, or may be set to an amount-of-light controlcharacteristic curve 158B where the difference between a highlight levelLh′ and the medium level Lm is smaller than with the amount-of-lightcontrol characteristic curve 158A, as indicated by the dot-and-dash linein FIG. 12. The amount-of-light control characteristic curve thusestablished is capable of preventing the gradation inversion phenomenonfrom occurring.

[0149] Modification 3:

[0150] Elimination of a tail produced in the recorded image due to achange in the amount of recording light P between the highlight level Lhand the medium level Lm.

[0151]FIG. 13 is illustrative of a tailing phenomenon. FIG. 13 shows, atA, a picture pattern 300 of an image that may possibly cause a tailingphenomenon. The picture pattern 300 includes a light image area 301corresponding to a highlight area in its trailing end (scanned earlier)in the main scanning direction X, a dark image area 302 following thelight image area 301 in the main scanning direction X and correspondingto a medium area (including a shadow area), and a light image area 303following the dark image area 302 in the main scanning direction X andcorresponding to a highlight area.

[0152]FIG. 13 shows, at B, the manner in which the data of the amount ofrecording light P generated from the light beam intensity correctingmemory 156 based on the recording duty cycle PDUTY detected by therecording duty cycle detector 154 changes with respect to the image daterepresenting the picture pattern 300.

[0153]FIG. 13 shows, at C, the manner in which the ideal laser power(the amount of recording light) applied to the photosensitive medium 32changes based on the amount of recording light P shown in FIG. 13 at B.According to the illustrated changes in the ideal laser power, the laserpower falls instantaneously from the highlight level Lh to the mediumlevel Lm at a position x0 and rises instantaneously from the mediumlevel Lm to the highlight level Lh at a position x2, depending on thedata of the amount of recording light P.

[0154]FIG. 13 shows, at D, changes in the actual laser power. Actually,the actual laser power has a response delay from the position x0 wherethe amount of recording light P changes, reaches the medium level Lm ata position x1, and reaches the highlight level Lh at a position x3.

[0155]FIG. 13 shows, at E, an ideal picture pattern 304 corresponding tothe picture pattern 300 shown in FIG. 13 at A, when it is recorded onthe photosensitive medium 32 with the ideal laser power shown in FIG. 13at C. FIG. 13 shows, at F, a picture pattern 306 with a tail when it isrecorded on the photosensitive medium 32 with the actual laser powershown in FIG. 13 at D. Each of the picture patterns 304, 306 is shownslightly wider than actual picture patterns in the auxiliary scanningdirection Y.

[0156] As shown in FIG. 13 at F, a dark image area 302A between thepositions x0 and x1 has its density highest at the position x0 andgradually reduced to a desired density of a dark image area 302B at theposition x1, resulting in a tailing phenomenon, i.e., a phenomenon inwhich a density gradation takes place though the picture pattern has aconstant density as indicated in the dark image area 302 in FIG. 13 atA. A light image area 303A between the positions x2 and x3 also suffersa tailing phenomenon where its density gradually decreases. However, thedensity change in the light image area 303A between the positions x2 andx3 is not distinct and cannot visually be perceived by human vision asthe density in the light image area 303A is originally low. Therefore,the dark image area 302A suffering the tailing phenomenon between thepositions x0 and x1 causes a major problem. The density of the darkimage area 302B is of a constant level which is the same as the densityof the dark image area 302.

[0157]FIG. 14 shows an exposure signal controller 70E arranged toeliminate the tailing phenomenon described above with reference to FIG.13 at A-F.

[0158]FIG. 15 is illustrative of an algorithm for eliminating a tailingphenomenon using the exposure signal controller 70E. FIG. 15 shows, atC, the picture pattern 300 of an image that may possibly cause a tailingphenomenon as shown in FIG. 13 at A.

[0159] As shown in FIG. 14, the exposure signal controller 70E includes,in addition to the components of the exposure signal controller 70 shownin FIG. 3, a preceding recording duty ratio detector 154A for detectinga recording duty ratio at a position scanned later in the main scanningdirection, i.e., a preceding position in the main scanning direction,than the recording duty cycle detector (referred to as “presentrecording duty cycle detector”) 154 which detects a present recordingduty cycle PDUTY, and an edge detector 312 serving as an intensitymodulation correcting means for outputting a control signal Se to shiftthe common contact of a preceding amount-of-light changing switch 310 asa preceding amount-of-light changing means for detecting, prior to arecording process, the position x0 of an edge (changing point) where thedensity changes from the light image area 301 to the dark image area302, from a change in the preceding recording duty cycle ADUTY outputtedfrom the preceding recording duty ratio detector 154A and the highlightlevel (data) Lh outputted from the light beam intensity correctingmemory 156, and changing the amount of recording light P from thehighlight level Lh to the medium level Lm when the position of the edgex0 is detected.

[0160] The preceding amount-of-light changing switch 310 has a fixedcontact connected to the light beam intensity correcting memory 156 foroutputting the highlight level Lh or the medium level Lm, and anotherfixed contact connected to the CPU 148 for outputting the medium levelLm as a constant level.

[0161] In FIG. 14, the readout address of the determining area ADarea inan image for detecting the preceding recording duty cycle ADUTY with thepreceding recording duty ratio detector 154A is specified by the linememory controller 152 or the CPU 148. As shown in FIG. 15 at A, thedetermining area needs to be a determining area (preceding determiningarea) ADarea that is scanned later than the determining area Darea ofthe recording duty ratio detector 154 by a time interval (preceding timeinterval) ds.

[0162] If the present recording position is a most trailing position inthe main scanning direction X of the determining area Darea, asindicated by the downward arrow in FIG. 15 at B, then in order to detectthe position x0 where the density changes from the highlight level tothe medium level, the time interval ds is required to be longer than atleast the response delay x1−x0 shown in FIG. 13 at D. For example, thetime interval ds may be set to a value ranging from 1 to 2 times theresponse delay x1−x0.

[0163] If the time interval ds is set to the response delay x−x0, thenwhen an image is recorded at the highlight level Lh based on therecording duty ratio PDUTY outputted from the recording duty ratiodetector 154, i.e., when the recording duty ratio PDUTY is smaller than6%, or when the preceding recording duty ratio ADUTY is set to a valueequal or greater than 6% by the preceding recording duty ratio detector154A (the example shown in FIG. 12), or the recording duty ratio PDUTYis equal to or greater than 25% (the example shown in FIG. 5), thecontrol signal Se from the edge detector 312 is applied to shift thepreceding amount-of-light changing switch 310 to supply the medium levelLm from the CPU 148 to the D/A converter 159. That is, the commoncontact of the preceding amount-of-light changing switch 310 is shiftedfrom the light beam intensity correcting memory 156 to the CPU 148 toconnect the input terminal of the D/A converter 159 to the CPU 148.

[0164] As indicated at a position x-1 in FIG. 15 at D, though the lightimage area 301 is being scanned, the amount of recording light P isreduced from the highlight level Lh to the medium level Lm at apreceding time. At this time, as shown in FIG. 15 at E, the actual laserpower is gradually reduced.

[0165] At a position x2 where the image density changes from the darkimage area 302 to the light image area 303, the edge detector 312applies the control signal Se to connect the input terminal of the D/Aconverter 159 to the light beam intensity correcting memory 156 when thepresent recording duty cycle PDUTY of the present recording duty cycledetector 154 becomes a value to output the highlight level from thelight beam intensity correcting memory 156, regardless of the value ofthe preceding recording duty cycle ADUTY of the preceding recording dutycycle detector 154A.

[0166] In this manner, as shown in FIG. 15 at D, the amount of recordinglight P is changed from the medium level Lm to the highlight level Lh atthe position x2. The actual laser power gradually increases from theposition x2 as shown in FIG. 15 at E.

[0167] With the laser power thus controlled, as indicated by a picturepattern 308 in FIG. 15 at F, the density of a dark image area 302Cbetween the positions x0 and x1 does not change as compared with thepicture pattern 306 shown in FIG. 13 at F. In the picture pattern 308shown in FIG. 15 at F, the density is slightly reduced in a light imagearea 301A between the positions x−1 and x0 and a light image area 303Abetween the positions x2 and x3 because the actual laser power isreduced as shown in FIG. 15 at E. However, since the density of thelight image areas 301, 303 of the original image 300 is low, thereduction in the density cannot visually be perceived. Accordingly, thetailing phenomenon can virtually be eliminated.

[0168] Specifically, in the exposure signal controller 70E shown in FIG.14, the preceding determining area ADarea is provided in addition to thedetermining area Darea, and an image in a medium halftone % area whereimage irregularities are noticeable, i.e., the dark image area 302, isdetermined in advance. The amount of light starts being reduced beforethe light beam L reaches the edge position x0. In this manner, the slowresponse in controlling the amount of light is compensated for and willnot be visually recognized as image irregularities.

[0169] In all of the embodiments described above, the recording dutycycle detector 154 comprises a digital circuit. However, the recordingduty cycle detector 154 and the preceding recording duty cycle detector154A may comprise an analog circuit.

[0170]FIG. 16 shows an exposure signal controller 70F where therecording duty cycle detector 154 is replaced with a low-pass filter inthe form of an analog circuit. For the sake of brevity, a single lightbeam L is shown in FIG. 16. However, a plurality of light beams such asthree light beams may be employed.

[0171] In FIG. 16, a binary image signal IS1 which has a value 1 or 0 issupplied from the parallel-to-serial converter 209 to a low-pass filter314 and also to an input terminal of a multiplier 221 of an AOM driver72B. The AOM driver 72B differs from the AOM driver 72 shown in FIG. 9in that the voltage control gain varying circuit 218 and the multiplier221 are switched around. The positions of the voltage control gainvarying circuit 218 and the multiplier 221 are of design matter, andhence can be changed as desired in design.

[0172] The low-pass filter 314 has its output value increased dependingon the probability of appearance of the value 1 of the binary imagesignal IS1. Specifically, the low-pass filter 314 outputs a signal SDUTYproportional to the recording duty cycle PDUTY of the binary imagesignal IS1. The signal SDUTY from the low-pass filter 314 is compared bya comparator 320, which outputs a binary signal to control the gain ofthe voltage control gain varying circuit 218 in a binary fashion.

[0173] With the low-pass filter 314 used as the recording duty cycledetector, the circuit arrangement is highly simplified because nocounting means and memory are required. The comparator 320 may bereplaced with an amplifier whose amplifying characteristic curveapproximates the amount-of-light control characteristic curve 158 shownin FIG. 5.

[0174] The time constant of the low-pass filter 314 possibly tends toproduce beats due to interference with the halftone dot pattern. Toavoid such beats, as shown in FIG. 17, a random number Rd may begenerated by a random number generator 322 and a D/A converter 324 andadded to the signal from the low-pass filter 314 by an adder 326connected between the low-pass filter 314 and the comparator 320.

[0175] According to the present invention, as described above, therecording duty cycle of an image to be recorded on the photosensitivemedium is detected before the image is actually recorded on thephotosensitive medium, and the amount of light applied to record theimage is controlled based on the detected recording duty cycle.Therefore, the amount of recording light can be corrected highlyaccurately.

[0176] For example, if the photosensitive medium is of such a naturethat an area irradiated with light remains as an image, then theintensity of a light beam applied thereto is modulated to a higher levelin a highlight area of the image. Therefore, the plate wear resistanceof the photosensitive medium in the highlight area is maintained at asufficient level, image irregularities are prevented from occurring.

[0177] Thus, the intensity of the light beam can be controlled withhigher accuracy in smaller ranges than halftone %.

[0178] The amount of recording light can be corrected without the needfor a correcting memory table. Since no correcting memory table is used,the image recording apparatus is simple in arrangement and low in cost.

[0179] Although certain preferred embodiments of the present inventionhave been shown and described in detail, it should be understood thatvarious changes and modifications may be made therein without departingfrom the scope of the appended claims.

What is claimed is:
 1. An apparatus for recording an image by scanning aphotosensitive medium with a light beam generated based on an imagesignal, comprising: recording duty cycle detecting means for detecting arecording duty cycle of an image to be recorded on the photosensitivemedium based on the image signal; and light beam intensity modulatingmeans for modulating the intensity of the light beam based on thedetected recording duty ratio.
 2. An apparatus according to claim 1 ,wherein said photosensitive medium is of such a nature that an areairradiated with light remains as an image, and said light beam intensitymodulating means comprises means for modulating the intensity of thelight beam to a higher level in a highlight area of the image.
 3. Anapparatus according to claim 2 , wherein said highlight area comprises ahighlight area smaller than 25% of all gradations of the image recordedon said photosensitive medium.
 4. An apparatus according to claim 1 ,wherein said recording duty cycle detecting means comprises a low-passfilter.
 5. An apparatus according to claim 1 , wherein said recordingduty cycle detecting means comprises means for detecting a recordingduty cycle corresponding to a given area in the image recorded on saidphotosensitive medium.
 6. An apparatus according to claim 5 , furthercomprising: random number applying means for varying the position of thegiven area in the image with a random number.
 7. An apparatus accordingto claim 5 , further comprising: random number applying means forvarying the size of the given area in the image with a random number. 8.An apparatus according to claim 5 , wherein said light beam intensitymodulating means comprises random number applying means for applying arandom number to the detected recording duty cycle, and means formodulating the intensity of the light beam based on the recording dutycycle to which the random number is applied by said random numberapplying means.
 9. An apparatus according to claim 1 , wherein saidlight beam comprises a plurality of light beams for simultaneouslyscanning said photosensitive medium to record the image thereon, andwherein said recording duty ratio detecting means comprises a pluralityof recording duty ratio detecting means associated respectively withimages recorded on the photosensitive medium based on respective imagesignals to generate said light beams, and said light beam intensitymodulating means comprises a plurality of light beam intensitymodulating means associated respectively with recording duty ratiosdetected by said recording duty ratio detecting means.
 10. An apparatusaccording to claim 1 , wherein said light beam comprises a plurality oflight beams for simultaneously scanning said photosensitive medium torecord the image thereon, and wherein said recording duty ratiodetecting means comprises means for determining an average recordingduty ratio of images recorded on the photosensitive medium based onrespective image signals to generate said light beams, and said lightbeam intensity modulating means comprises means for modulating thebrightnesses of said light beams based on said average recording dutyratio.
 11. An apparatus for recording an image by scanning aphotosensitive medium which is fed in an auxiliary scanning direction,with a light beam generated based on an image signal in a main scanningdirection substantially perpendicular to said auxiliary scanningdirection, comprising: present recording duty cycle detecting means fordetecting a present recording duty cycle of an image to be recorded onthe photosensitive medium based on the image signal; light beamintensity modulating means for modulating the intensity of the lightbeam based on the detected present recording duty ratio; precedingrecording duty cycle detecting means for detecting a preceding recordingduty cycle of the image at a position scanned later than said presentrecording duty cycle detecting means in the main scanning direction; andintensity modulation correcting means for comparing the detectedpreceding recording duty cycle and the detected present recording dutycycle to correct the modulation of the intensity of the light beam withsaid light beam intensity modulating means.
 12. An apparatus accordingto claim 11 , wherein said photosensitive medium is of such a naturethat an area irradiated with light remains as an image, and said lightbeam intensity modulating means comprises means for modulating theintensity of the light beam to a higher level in a highlight area whichis smaller than 25% of all gradations of the image.
 13. An apparatusaccording to claim 12 , wherein said preceding recording duty cycle isof a value corresponding to the highlight area which is smaller than 25%of all gradations of the image, and said present recording duty cycle isof a value corresponding to an area except the highlight area which issmaller than 25% of all gradations of the image, and wherein saidintensity modulation correcting means comprises means for correcting themodulation of the intensity of the light beam to cause the intensity ofthe light beam to return from a given position in the highlight area toa normal intensity.
 14. A method of recording an image by scanning aphotosensitive medium with a light beam generated based on an imagesignal, comprising the steps of: detecting a recording duty cycle of animage to be recorded on the photosensitive medium based on the imagesignal; and modulating the intensity of the light beam based on thedetected recording duty ratio.
 15. A method according to claim 14 ,wherein said photosensitive medium is of such a nature that an areairradiated with light remains as an image, and said step of modulatingthe intensity of the light beam comprises the step of modulating theintensity of the light beam to a higher level in a highlight area whichis smaller than 25% of all gradations of the image.